Amazing to discover this forum.. so helpful!
Looking at starting a first build - 6" system (existing flue is 6" and space is limited).
Couple of complete newbie questions -
1. Should a 6" system be up to the job? We live in a north facing house in south-west UK - it dips below 32degF on and off for about 3 months.. very rarely ever lower than 26degF at night. House is compact - 3 bed bungalow with open plan up to a loft conversion. Well insulated roof and double glazed, poor 1950s external walls. Currently heated completely fine by a 6.5KW Morso woodburner (using LOTS of wood though!).
I have about 13' of space for the length of the bench.
2. Is there a reason you can't build a U-shaped bench?! Planning to build ours round an alcove, but can't find any examples. I read that each 90 degree bend = subtract 5' from flue length which should hopefully still leave us fine to use a 13' bench.
Initally thought 8" system but that would mean re-doing the exsiting chimney (lots of money and work) and ending up with a short L-bench that doesn't work nearly as well from a use of space / aesthetic / girlfriend perspective!
Hi Pete; Welcome to Permies! And A Big Welcome to the wonderful world of Rocket Science!
Short answer is yes a 6" system should keep you plenty warm.
The next question is what style of RMH were you thinking of?
A traditional J tube or a batch box style?
You mention a piped bench , are you aware you can have a bell bench?
Are you aware that you can have a brick/ or metal bell of any shape that fits your space?
So many choices and lots of information available.
Here are a couple of links for you to check out.
The first is Matt Walker's website. Matt has a riserless core design that he incorporates into cook stoves , waterheaters / BBQ's
https://walkerstoves.com/index.html Next is Peter Bergs Batch Rocket site. Peter is a master builder who has fine tuned batchboxes to their optimum output.
https://batchrocket.eu/en/ Read all about them here at Permies and over at Walker stoves and Batch rocket.
Come back often and ask all the questions you like!
Thanks loads for the info.. bell benches, batch boxes and riserless cores! My mind is blown! And brillliant that I don't have to cut yet another hole in the roof!
I suppose the main question I have now is why would anyone use a piped bench system or a J tube?!
Is it really true that the reduced mass in a bell bench (from having it empty inside) is compensated for by the gases hanging around longer before sinking out of the bottom? I'm struggling to see how 2" slabs as a bell bench top can be as effective as a whole (piped) bench full of cob...
I love the ceramic fibre board core template idea (I'll see if Matt Walker has a board template for a batch box). It looks like Peter Berg is using brick (or cast) which we're more than happy to use too of course, but are there any cons to using board? Cost of board vs. firebrick seems to be roughly similar here..
I'm assuming there are some rules about proportions, etc when it comes to the bell.. I can't just hot in top and cold out bottom of any size / shape I like surely?!
I'll keep looking for designs.. maybe people are building J-tube + piped bench because of the wealth of well-tested designs available.
Matt Walker's designs seem to incorporate cook stoves which we don't need (yet!).. I'll keep looking for a way to incorporate metal into a riserless core version (we'd love some quick heat), otherwise maybe we'll go with the typical barrel over riser arrangement and then manifold out into a bell...
I agree about the piped bench - a bell is so much more flexible that it seems like a no-brainer. There is no reason a bell has to be less massive than a piped bench, it merely can be if desired. My (vertical) bell has 2 1/2" of brick and 6"+ of cob for a total of about 9" thick, and plenty massive. It also has steel access panels that give some instant radiant heat.
A J-tube is simpler to build than a batch box, and more forgiving of minor alterations. It is more hands-on than a batch box, and might be better at fine-tuning the amount of heating in a mild climate. That is probably a matter of opinion though.
Bell proportions really are as flexible as it sounds. As long as you use the correct internal surface area (floor not included), it can be pretty much any shape that allows the gases to move around.
I see we have another mad rocket scientist in the making here!
Keep wearing your regular clothing until your Lab coat and dorky glasses arrive... mail service is slow with the current medical problems!
So why would someone still make a J tube and piped mass?
To start, a J tube is so easy to build and a batch box requires some metal fabrication. Many choose to start with the J and later they will upgrade to a Batch.
Now in my opinion, a piped mass is outdated, and bells are a much better idea!
But many folks worry that a bell surely would leak carbon monoxide into their home. Other than maybe a little smoke at start up nothing leaks indoors.
Installing a "bypass" gate eliminates any cold chimney (draft) problems.
Is an empty bell as warm as a solid piped mass? There is some debate about which is longer lasting.
I think that the solid mass once it is completely warm would hold heat a little longer... but not enough for me to build one anymore.
We lived with an 8" J tube and solid mass for 7 years, They work outstanding but you must pop in to add wood every 45 minutes or so.
Our is in an uninsulated greenhouse in Northern Montana, we keep it heated all winter with no fire at all from 9pm - 7am. -10 F outdoors and 40F indoors in the morning!
Feeding it all day was no big deal, just another chore.
Last year I removed the 8" J tube and replaced it with a 6" batchbox. I still utilize the 8" pipes and solid core, but now we have a much nicer, hotter burning stove that will last 2+ hrs before needing wood added.
My shop had an 8" J tube going into a large brick bell. I converted it into a 7" batchbox! I love it, Much warmer with a batch and less adding wood.
Next ceramic board (CFB) versus fire brick)
In my opinion the CFB works super as long as it is nowhere near the wood.
The CFB is to easy to damage, any wood that bounces off a wall or roof WILL dent/ damage the CFB. Let that happen a dozen times and suddenly you need to replace your CFB.
When a batchbox is up flying (we call them Dragons) it is very similar to a forge... white hot, when you open the door to add wood either you wear a welding glove OR you quickly toss in wood and shut the door... that is when you will damage your CFB.
When you build with firebrick you have no issues with damage unless you are brutal.
But you must allow those bricks to get up to temperature before you can expect your Dragon to soar.
Rules about bell size.
Yes there are definite size limitations listed on Peter Bergs site. ISA internal surface area of the bell.
However by installing a bypass to heat the chimney and start the draft you can exceed those ISA numbers.
With several bypasses installed a person could have an enormous series of bells. Most of us stick close to Peters numbers but we still install a bypass to eliminate any startup draw issues.
Matt's tiny cookstove with the glass top really throws quick heat just like a barrel and it can be plumbed directly into a bell or bench to mass heat or it can be run as just a stove for those days when you just want to take the chill off not bake yourself out of doors!
Thanks both for the really useful info… photos are great!
Sold on the bell!
Given it’s my first go I’m finding the idea of detailed plans really appealing!
Looking at Dan Walkers plans we’re thinking the batch RMH is probably favourite - although would be great to incorporate some metal somewhere for instant heat - intrigued by your access panels Glenn, don’t suppose you have a photo?
We are short on space, so I’m trying to keep the footprint of the main section down - again Dan’s plan looks more compact than a typical steel drum..
Ideally we’d like a longer bench than Dan’s example, but realise about the issue of ISA - I understand we could see how it goes over time with a bypass (def need to work that in!).. build the extra section and break into it if we manage to get everything else working!
I don’t fully understand how ISA is calculated (ie. which bits of the RMH are included) and whether any mods could be made to Dan’s plan to reduce anywhere to be able to lengthen the bench? Having said that I am definitely keen to stick as close to the plans as possible..
Lastly, my gf does like to sit in front of a fire. I’m assuming from the chat about forges and welding gloves that she won’t be sitting in front of the batch with the door open much?!
Consider the Internal Surface Area to be all of the flat planes or curved areas (half barrel benches) that face your hot gases, with the exception of the core and the riser, any insulated surfaces, and of course, the floor of the system.
Hi Pete, I assume your referring to Matt Walker not Dan Walkers plans? Also, with the purchase of Matt's plans, there is unlimited free consulting. He is a super nice guy and will help anyone out regardless of plan purchase but really helps to support him. Hopefully his "Stove Chat" will start up again in the fall where anyone can join in and get all geeky about anything rocket and wood stove related.
Argh.. yes indeed I meant Matt! Got to try to get to bed earlier!! There's so much to read about stoves though...
Thanks Thomas for clearing that up about ISA - really helpful
Hi Gerry - will definitely be supporting Matt - plans or not - looking likely at the moment that I'll attempt a variation on his Tiny stove (riserless core so low profile and has a bypass) and he's got back to me by email already - super-helpful like you say!
I'll post back some plans and hopefully pics if and when we get to building.
One general question I had was about the ISA of benches as bells and whether you can go long with the bench by decreasing the size of the void inside? Maybe by double skin of brick or even adding a load of cob in there.. is it a good idea to be getting as much mass as possible into these things anyway?
I saw Matt saying that the majority of the heat is absorbed into the top of the bell so perhaps makes sense to focus on adding mass there?
As I understand it once you get over a certain bell volume there's just not enough temp left in the gases to create a suitable draw.. is it similar with mass, or can you just keep adding mass and have it work fine, but just not expect it to get as hot?
Pete, theoretically you can add all the mass you want. And yes, if you don't want a sweaty bottom, maybe it is a good idea to concentrate the bulk of it on top. The important consideration is that heat travels through mass slowly, the more dense the mass, the slower it moves, so you have to keep that in mind because while a massive bench will heat a space for a long time once charged, it may also tend to take several hours to begin dissipating heat to the room. But in short, it is the ISA that is the limiting factor to your system. The flue gases are dumb and don't know how much mass there is. They only interact with the ISA.
More mass will indeed store more heat for longer, but there is a limit to practicality. A rule of thumb is that cob transmits heat at around an inch per hour, so if you have an 8" thick bell wall, you will not be getting the full heat for 8 hours after starting the fire.Brick and especially hard firebrick transmit heat faster than that; the 2 1/2" thick firebrick on edge of my upper bell, where it is exposed until I build the stone chimney, gets warm within a half hour and too hot to touch in an hour or two depending on how hot I make the fire. Meanwhile, the flat bell walls at 9" total thickness don't start to get warm for a couple of hours or more, but stay warm for 24 hours after the fire is out. The corners, even with a 4" or so radius, are effectively 12" or so thick and stay cold for 5 hours or more, and only ever feel really warm if I am burning long for subzero nights.
That's great - thanks for the help!
Really handy to see your experience of the different materials, thicknesses and dissipation times too Glenn!
I figure I should start with a simple empty brick bench with some heavy slabs as lids to get the system up and running. From there I can experiment and add more mass as needed...
Thanks again everyone for all the help.. really appreciated. I'm off to start piling up firebricks and setting fire to things...
I'm still deciding between using ducts in mass or a bell. If I can make it to the RMH jamboree event this fall that would certainly help, talking to the experts and hands-on experience with building and repair!
I'm leaning towards a bell, and bridging the space I was thinking I could make cob/adobe bricks a few days in advance of the build, and then use those as a central support piece in the middle so the top pieces could bridge from front to middle, and from middle to back of the bench. Otherwise I'd be concerned that my big butt would be too much weight as I planned to use cob and on-site rocks to build the bell.
I still recall the delight when visiting Cob Cottage Company in coastal Oregon in October several years ago, how the library RMH ran around dinner time for 45-60 minutes, and 12 hours later we would come in to eat breakfast and it was 35F outside, and inside the cob building it was probably 65+, and sitting on the bench (which is U shaped) was even warmer.
I'm not quite a lumberjack, but that's OK, I sleep all night and I dream all day; I'll coppice trees, I'll grow my food, and compost poo and pee! With a well and off-grid solar, it's a permies life for me! https://www.youtube.com/watch?v=FshU58nI0Ts
Mark, as far as "bridging the gap" maybe you might consider placing support blocks along the inside lengths of your bench and bridging the gap with two lengths of angle iron placed back to back. This will offer support for blocks to be laid across the span. If, like me, you are planning on using a lot of solid 4"x 8" x 16" solid concrete block for your bench, this could come in very handy. There are a lot of sizes of pre-fab block to work with. Kinda like playing with my kid's Lego's. At $70 per ton, it's not a bad way to go.
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